A distributed Bragg reflector laser (hereinafter referred to as a “DBR laser.” DBR: Distributed Bragg Reflector) is used as a high-speed wavelength tunable laser by injecting an electric current into a DBR region serving as a distributed Bragg reflector and into a phase control region (Non-Patent Document 1). Once an electric current is injected into the DBR region and the phase control region, the refractive index of a waveguide core layer decreases due to a plasma effect. This allows shifting the lasing wavelength to its shorter wavelength side. The response speed of the wavelength tuning due to the plasma effect is extremely high, and is the order of 10−9 seconds in theory. However, heat is generated due to a resistance component of the semiconductor device when the electric current is injected, and makes the lasing wavelength change gradually. As a result, it takes approximately 10−3 seconds for the lasing wavelength to become stable. The wavelength drift due to the heat in the order of 10−3 seconds is extremely slower than the response speed of the wavelength tuning due to the plasma effect, which causes a serious problem of making the tuning speed slower. To solve this problem, a number of methods have been proposed (Patent Documents 1 and 2 as well as Non-Patent Documents 2 and 3).
To solve the above-described problem, calculated in Non-patent Document 2 is the temperature rise of the top surface of a device in the case where an electric current injected into the device is changed. However, this method is incapable of directly monitoring the junction voltage. In addition, this method requires investigation of the thermal resistance between an optical waveguide on a top surface of the device and a heat sink on a bottom surface of the device. For these reasons, this method is unsuitable for mass production.
In Non-patent Document 3, detailed control is periodically provided by use of an apparatus comprising an optical branching filter and a delay optical fiber, to prevent wavelength drifts. In practice, this method branches a part of light with a stable wavelength into two portions, and thereafter duplexes together the thus-branched portions of the light in a final stage with one of the two branched portions thereof being delayed. However, this method requires the quantity of inputted heat, heat capacity, and parameters for the heat exhaustion rate and the like called thermal resistance to be found beforehand in order to keep the temperature constant. Thus, this method requires various preparations to be made in order to provide precise control. Moreover, this method needs equipment such as the optical branching filter and the delay optical fiber in order to configure a light source, and is thus disadvantageous in cost as well.
In Patent Document 1, thermal compensation is provided by use of a thermal compensation controlling electrode. To this end, a value of an electric current injected into a thermal compensation region is determined by use of a correction coefficient. Patent Document 1 describes in Paragraph 0024 that this correction coefficient is automatically determined by an apparatus. However, determination of the correction coefficient requires an extraordinarily long time. This is because, for the determination of the correction coefficient, the parameters need to be determined on the basis of a result of monitoring the lasing wavelengths of the laser at the time of static drive and high-speed wavelength switching while frequently changing the thermal compensation current. Furthermore, the method is incapable of precise fitting since the fitting expression for controlling the laser includes neither a first-order term nor a constant term.
Thermal compensation is provided by use of a thermal compensation controlling electrode in Patent Document 2 as well. However, in Patent Document 2, neither a device resistance of a wavelength tuning region nor a device resistance of a thermal compensation region is included as a control element. As a result, this thermal compensation region needs to have a resistance equal to that of the wavelength tuning region. This requires the thermal compensation region and the wavelength tuning region to have the same shape and electric resistance. For this reason, this control method has a problem of being incapable of dealing with anything but a specific device, and of being accordingly incapable of preventing the wavelength drifts when the device resistance is different from one another. Furthermore, this control method requires the device manufacturing process to be pursued with high precision, with high uniformity, and with high reproducibility. As a result, this control method causes a problem of decrease in yields and increase in cost in the device manufacturing process. Moreover, this control method includes neither a second-order term nor a constant term in the fitting expression for controlling the laser, and is hence incapable of precise fitting.
As described above, the methods having so far been proposed bear various problems. Accordingly, no practical method has been proposed yet.
In the following, a configuration of a conventional DBR laser will be shown. In addition, a measurement result of wavelength drifts which occur due to heat will be shown. FIG. 27 shows a top view of a 2-section DBR laser as the conventional DBR laser, in which a total of two regions including an active region and a DBR region are illustrated with their respective simplified configurations. FIGS. 28A, 28B, 28C respectively show cross-sectional views of the DBR laser taken along a XXVIIIA-XXVIIIA line, a XXVIIIB-XXVIIIB line, and a XXVIIIC-XXVIIIC line in FIG. 27.
The conventional DBR laser shown in FIGS. 27, 28A, 28B and 28C includes: an active region 173 for oscillating a laser beam; and a DBR region 175 for shifting the wavelength of the laser beam. The active region 173 includes: an active layer 172 linearly formed on a substrate serving as a lower clad 171; and an upper clad 177 formed in a convex shape on the active layer 172. The DBR region 175 includes: a non-active layer 174 formed on the lower clad 171; a diffraction grating 176 formed in the top surface of a portion of the non-active layer 174, the portion and the active layer 172 being arranged in a straight line; and an upper clad 177 formed in a convex shape on the diffraction grating 176. With this configuration, an optical waveguide in each of the active region 173 and the DBR region 175 is formed in a mesa structure.
In addition, the conventional DBR laser includes an insulating film 178 which is formed on surfaces of the active layer 172, the non-active layer 174 and the upper clad 177 excluding the top surface of the upper clad 177. As electrodes, an active region electrode 179a, a DBR region electrode 179b and a lower electrode 180 are included. The active region electrode 179a is formed on the top surface of a portion, which constitutes the active region 173, of the upper clad 177. The DBR region electrode 179b is formed on the top surface of a portion, which constitutes the DBR region 175, of the upper clad 177. The lower electrode 180 is formed on the bottom surface of the lower clad 171. Moreover, an antireflection coating (hereinafter referred to as an “AR coating”) 181 is formed on a side end surface of the non-active layer 174, which constitutes the DBR region 175.
As described above, the conventional DBR laser comprises the active region 173 and the DBR region 175, and forms a laser cavity with the reflectance factor of approximate 30% of the cleavage end surface of the active layer 173, and the reflection structure of the DBR region 175. Thus, the conventional DBR laser oscillates a laser beam when an electric current is injected into the active region electrode 179a, while shifting the wavelength of the laser beam when an electric current is injected into the DBR region electrode 179b. 
FIG. 29 shows how wavelengths (frequencies) behave when the wavelengths are switched back and forth by changing a value of an electric current to be injected into the DBR region electrode 179b in the conventional DBR laser.
In this case, the value of the electric current injected into the DBR region electrode 179b is alternately switched between 20 mA and 53 mA every 4 milliseconds to thereby output wavelengths corresponding to 192.75 THz and 193.15 THz alternately. For clear understanding of how the wavelengths are drifted due to heat, FIGS. 30A and 30B respectively show behaviors of the wavelengths by magnifying the vertical axis scale of FIG. 29 to 10 GHz. The wavelength shown in FIG. 30A is drifted by approximately 2 GHz, whereas the wavelength shown in FIG. 30B is drifted by approximately 6 GHz. The wavelength drift shown in FIG. 30B which occurs due to the heat each time the electric current is switched from the lower to higher levels is larger than the wavelength drift shown in FIG. 30A.
The present invention has been conceived in consideration of the above-described problems. An object of the present invention is to provide a wavelength tunable semiconductor laser device, a controller for the same and a control method for the same, which are capable of preventing wavelength drifts attributable to a heat collaterally generated when a wavelength is tuned by use of a plasma effect.    [Patent Document 1] Japanese Patent No. 3168855    [Patent Document 2] Japanese Patent No. 3257185    [Non-patent Document 1] Tetsuhiko Ikegami, “Semiconductor Photonics,” Corona Publishing, Oct. 10, 1995, pp. 306-311.    [Non-patent Document 2] Nunzio P. Caponio et al., “Analysis and Design Criteria of Three-section DBR Tunable Lasers,” IEEE Journal on Selected Areas in Communications, August 1990, vol. 8, no. 2, pp. 1203-1213.    [Non-patent Document 3] Osamu Ishida et al., “Fast and stable Frequency Switching Employing a Delayed Self-Duplex (DSD) Light Source,” IEEE Photonics Technology Letters, January 1994, vol. 6, no. 1, pp. 13-16.    [Non-patent Document 4] Ishii Hiroyuki, “Doctoral dissertation: Research on Enhancing Performance of Wavelength Tunable Semiconductor Laser Device,” March 1999, Chapter 4.